Evaporation control using Rio Tinto’s floating modules on Northparkes Mine

Water is essential to the profitable operation of all mine sites. It is essential that minesite water balances be optimised to ensure that losses are reduced and the efficiency of the systems maximised. This is particularly the case for mines where water supply is limited. In response to the need for continual improvement in the management of water,Rio Tinto has been investigating the use of water storage covers to minimise the loss of waterthroughevaporation. Floating modulesdesigned by Rio Tinto were deployed at Northparkes Mine and their effectiveness in reducing evaporation losses was measured.Measurement of evaporation losses was achievedby using a massbalance approach to the water balance of an open storage. This required accurate measurement of the change in water level over periods of a day or less and the apportioning of this loss to either evaporation or seepage.The trial consisted of two storages located adjacent to eachother. One of the storages was covered with floating modules while the other remained uncovered (acted as a control). The water levels of both storages weremeasured using highly accurate pressure sensitive transducers. Seepage was estimated and removed from the water level data for both storages using statistical methods(linear correlations between the change in water level and calculated evapotranspiration), leaving evaporation as the sole cause of changes in water height. The evaporation losses from the covered and control storages were compared to determine the efficiency of the floating modulesinreducing evaporation.Rio Tinto‟s floating modules reduced evaporation by 85 %.The floating modules coveredapproximately 90% of the surface area of the storage. For the storages located at Northparkes Mine, this was equal to a saving of 3.6mm/day or 1,235 mmover the 346 day trial period. For every hectare of water surface, this is equivalent to a saving of 12.4ML.The percentage reduction changed during the year with a maximum reduction of 95%achieved during winter.When considering increasing the efficiency of water storages, other options in addition to evaporation covers must also be considered. These include seepage minimisation, storage design and siting, and storage management. In some situations, these options may provide greater gains in efficiency and be more economical.